1. Field of the Invention
This invention relates to NMR imaging methods and has particular application in methods of obtaining NMR imaging information from solid objects.
2. Description of the Related Art
The NMR imaging of solid materials has not been developed to the same level of sophistication as the NMR imaging of liquid and quasi-liquid materials. The principal difficulties with NMR imaging in solids arise because of the rapid rate of decay of the transverse magnetization. In other words, the characteristic spin-spin relaxation time, usually designated T.sub.2, is short. To produce sufficient spatial localization of the selected nuclear spins in an object, large magnetic field gradients must be superimposed, which increases the rate of signal decay even further. In such circumstances much of the signal is lost in the dead time of the apparatus following the application of an rf excitation pulse. Also, the excitation pulse itself needs to be of large amplitude to excite resonance. Furthermore, because in solids the characteristic spin-lattice relaxation time T.sub.1 is much longer than T.sub.2, a considerable time interval of several T.sub.1 s, must elapse before an experiment can be repeated.